Steering control apparatus

ABSTRACT

A steering control apparatus includes a direct current power source, a three-phase alternating current motor, and a motor driving circuit. An emergency switching element is provided on at least two phases of a three-phase power supply line connected to the three-phase alternating current motor within the motor driving circuit, and the emergency switching element is turned off when an abnormality occurs such that the motor driving circuit is disconnected from the three-phase alternating current motor. The emergency switching element is a MOSFET, and the MOSFETs are provided in pairs in each of the two phases of the three-phase power supply line. Further, parasitic diodes of the pairs of MOSFETs are disposed in opposite orientations to each other.

INCORPORATION BY REFERENCE

This application is a division of and is based upon and claims thebenefit of priority under 35 U.S.C. §120 for U.S. Ser. No. 12/427,983,filed Apr. 22, 2009, which claims the benefit of priority under 35U.S.C. §119 from Japanese Patent Application No. 2008-112011, filed Apr.23, 2008, the entire contents of each of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a steering control apparatus for assistingsteering of a steering wheel by driving a three-phase alternatingcurrent motor, that is referred to hereafter as a “steering assistmotor”, serving as a driving source of an electric power steeringapparatus.

2. Description of the Related Art

In this type of steering control apparatus, when the steering assistmotor cannot be driven due to an abnormality, the steering assist motoris rotated by the steering of the steering wheel so as to function as apower generator, and as a result, a battery as direct current powersource of a vehicle, for example, is charged. Steering resistancegenerated at this time includes both frictional resistance from amovable portion and resistance required to generate power by convertingkinetic energy into electric energy. The resistance required to generatepower is referred to hereafter as “power generation resistance”.Meanwhile, in a conventional steering control apparatus, a mechanicalswitch is provided on a power supply line connected to the steeringassist motor. In this steering control apparatus, the mechanical switchis turned off when an abnormality occurs such that a motor drivingcircuit provided in the steering control apparatus is disconnected fromthe steering assist motor. As a result, the power generation resistanceis removed from the steering resistance, and thus a reduction insteering resistance when an abnormality occurs can be achieved such asJapanese Patent Application Publication No. 2005-199746“JP-A-2005-199746”.

However, a malfunction may occur in the mechanical switch due to theinfiltration of foreign matter.

SUMMARY OF THE INVENTION

An object of the invention is to provide, at low cost, a steeringcontrol apparatus capable of disconnecting a motor driving circuit froma three-phase alternating current motor reliably when an abnormalityoccurs.

A steering control apparatus according to a first embodiment of theinvention includes: a direct current power source installed in avehicle; a three-phase alternating current motor serving as a drivingsource of an electric power steering apparatus; a motor driving circuitprovided between the direct current power source and the three-phasealternating current motor so as to generate a three-phase alternatingcurrent from an output of the direct current power source and conductthe generated three-phase alternating current to the three-phasealternating current motor; an emergency switching element which isprovided on at least two phases of a three-phase power supply lineconnected to the three-phase alternating current motor within the motordriving circuit, and which is turned off when an abnormality occurs suchthat the motor driving circuit and the three-phase alternating currentmotor are disconnected, wherein the emergency switching element is a(metal-oxide-semiconductor field-effect transistor) MOSFET, the MOSFETsare provided in pairs in each of the two phases of the three-phase powersupply line, and parasitic diodes of the pairs of MOSFETs are disposedin opposite orientations to each other; and an emergency turn offcontrol unit for sequentially turning off the MOSFETs through which nocurrent is flowing or the MOSFETs through which a current oriented toturn the parasitic diodes on is flowing, the emergency turn off controlunit turn off all of the MOSFETs when an abnormality occurs.

In the steering control apparatus according to the first embodiment, theemergency switching elements provided on the power supply lines of themotor driving circuit for conducting a current to the three-phasealternating current motor serving as the driving source of the electricpower steering apparatus are MOSFETs, and do not therefore malfunctionwhen infiltrated by foreign matter, in contrast to a mechanical switch.In other words, the motor driving circuit and the three-phasealternating current motor can be disconnected reliably when anabnormality occurs.

Further, in the first embodiment, the MOSFETs serving as the emergencyswitching elements are provided in pairs in each of the two phases ofthe three-phase power supply line, and the parasitic diodes of theMOSFETs are disposed in opposite orientations to each other. Therefore,when the emergency switching elements are all turned off, a current nolonger flows through the two phases of the three-phase power supplyline, regardless of orientation. Accordingly, a conductive closedcircuit partially including a three-phase phase circuit of thethree-phase alternating current motor ceases to exist, and thereforepower generation resistance of the three-phase alternating current motoris removed, enabling a reduction in steering resistance when anabnormality occurs.

When an abnormality occurs during conduction of the three-phasealternating current between the motor driving circuit and thethree-phase alternating current motor, the MOSFETs through which nocurrent is flowing or the MOSFETs through which a current oriented toturn the parasitic diodes on is flowing are sequentially turned off suchthat all of the emergency switching elements are turned off. Here, evenwhen the MOSFETs through which a current oriented to turn the parasiticdiodes on is flowing are turned off, rapid current blocking does notoccur, and therefore a spike voltage is not generated. Needless to say,a spike voltage is not generated when the MOSFETs through which nocurrent is flowing are turned off. Hence, there is no need to useexpensive MOSFETs that can withstand a spike voltage as the emergencyswitching elements, and therefore a reduction in cost can be achieved.In other words, according to the first embodiment, a steering controlapparatus that is capable of disconnecting a motor driving circuit and athree-phase alternating current motor reliably when an abnormalityoccurs can be provided at low cost.

A steering control apparatus according to a second embodiment of theinvention includes: a direct current power source installed in avehicle; a three-phase alternating current motor serving as a drivingsource of an electric power steering apparatus; a motor driving circuitprovided between the direct current power source and the three-phasealternating current motor so as to generate a three-phase alternatingcurrent from an output of the direct current power source and conductthe generated three-phase alternating current to the three-phasealternating current motor; an emergency switching element which isprovided on a three-phase power supply line connected to the three-phasealternating current motor within the motor driving circuit, and which isturned off when an abnormality occurs such that the motor drivingcircuit and the three-phase alternating current motor are disconnected,wherein the emergency switching element is a MOSFET, the MOSFET isprovided singly in all three phases of the three-phase power supplyline, and parasitic diodes of all of the MOSFETs are disposed in anidentical orientation relative to the three-phase alternating currentmotor; and an emergency turn off control unit for sequentially turningoff the MOSFETs through which no current is flowing or the MOSFETsthrough which a current oriented to turn the parasitic diode on isflowing, the emergency turn off control unit turn off all of the MOSFETswhen an abnormality occurs.

In the steering control apparatus according to the second embodiment ofthe invention, the emergency switching elements provided on the powersupply lines of the motor driving circuit for conducting a current tothe three-phase alternating current motor serving as the driving sourceof the electric power steering apparatus are MOSFETs, and do nottherefore malfunction when infiltrated by foreign matter, in contrast toa mechanical switch. In other words, the motor driving circuit and thethree-phase alternating current motor can be disconnected reliably whenan abnormality occurs.

Further, in the second embodiment, the MOSFET serving as the emergencyswitching element is provided singly in all three phases of thethree-phase power supply line, and the parasitic diodes of the MOSFETsare disposed in an identical orientation relative to the three-phasealternating current motor. Hence, when the emergency switching elementsare all turned off, a conductive closed circuit partially including athree-phase phase circuit of the three-phase alternating current motorceases to exist, and therefore power generation resistance of thethree-phase alternating current motor is removed, enabling a reductionin steering resistance when an abnormality occurs.

When an abnormality occurs during conduction of the three-phasealternating current between the motor driving circuit and thethree-phase alternating current motor, the MOSFETs through which nocurrent is flowing or the MOSFETs through which a current oriented toturn the parasitic diodes on is flowing are sequentially turned off suchthat all of the emergency switching elements are turned off. Here, evenwhen the MOSFETs through which a current oriented to turn the parasiticdiodes on is flowing are turned off, rapid current blocking does notoccur, and therefore a spike voltage is not generated. Needless to say,a spike voltage is not generated when the MOSFETs through which nocurrent is flowing are turned off. Hence, there is no need to useexpensive MOSFETs that can withstand a spike voltage as the emergencyswitching elements, and therefore a reduction in cost can be achieved.In other words, according to the second embodiment, a steering controlapparatus capable of disconnecting a motor driving circuit and athree-phase alternating current motor reliably when an abnormalityoccurs can be provided at low cost.

In the steering control apparatuses according to the first and secondembodiments, a current sensor may be provided in the motor drivingcircuit, and the emergency turn off control unit may determine whetheror not the MOSFETs can be turned off on the basis of the three-phasealternating current detected by the current sensor.

In the above embodiment, the presence and orientation of a current inthe respective power supply lines can be determined on the basis of adetection value of the current sensor. The determination as to whetheror not the MOSFETs can be turned off, or in other words whether or not aMOSFET is a MOSFET through which no current is flowing or a MOSFETthrough which a current oriented to turn the parasitic diodes on isflowing, can then be made in accordance with the presence andorientation of the current.

Further, in the steering control apparatuses according to the first andsecond embodiments, a rotation position sensor may be provided fordetecting a rotation position of the three-phase alternating currentmotor, and the emergency turn off control unit may determine whether ornot the MOSFETs can be turned off on the basis of a detection valuedetected by the rotation position sensor.

In the above embodiment, a current phase of the current of each phase ofthe three-phase alternating current varies according to the rotationposition of the three-phase alternating current motor, and therefore thepresence and orientation of a current in the respective power supplylines can be determined on the basis of a detection value of therotation position sensor provided in the three-phase alternating currentmotor. The determination as to whether or not the MOSFETs can be turnedoff, or in other words whether or not a MOSFET is a MOSFET through whichno current is flowing or a MOSFET through which a current oriented toturn the parasitic diodes on is flowing, can then be made in accordancewith the presence and orientation of the current.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages, and technical and industrial significance ofthe invention will be described in the following detailed description ofexample embodiments of the invention with reference to the accompanyingdrawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a conceptual view of a vehicle installed with a steeringcontrol apparatus according to a first embodiment of the invention;

FIG. 2 is a circuit diagram of the steering control apparatus;

FIGS. 3A-3B are a circuit diagram showing connections between a motordriving circuit and a motor;

FIGS. 4A-4B is a circuit diagram showing connections between the motordriving circuit and the motor;

FIG. 5 is a flowchart of an emergency power supply stopping program;

FIG. 6 is a circuit diagram showing connections between a motor drivingcircuit and a motor according to a second embodiment; and

FIG. 7 is a flowchart of an emergency power supply stopping program.

DETAILED DESCRIPTION OF EMBODIMENTS First Embodiment

A first embodiment of the invention will be described below on the basisof FIGS. 1 to 5. FIG. 1 shows a vehicle 10 installed with an electricpower steering apparatus 11. The electric power steering apparatus 11includes a shaft 13 that is provided between steered wheels so as toextend in a left-right direction of the vehicle 10, and the shaft 13provided between the steered wheels is inserted into a core of a tubularhousing 15 fixed to a vehicle main body 10H. Further, the two ends ofthe shaft 13 provided between the steered wheels are connected torespective steered wheels 12, 12 via tie rods 14, 14.

A three-phase alternating current motor 19 (to be referred to simply asa “motor 19” hereafter) serving as a driving source of the electricpower steering apparatus 11 is a three-phase alternating current motorhaving a hollow tubular structure, for example. A stator 20 of the motor19 is fixed within the tubular housing 15, and the shaft 13 providedbetween the steered wheels penetrates a hollow part of a rotor 21. Aball nut 22 fixed to an inner surface of the rotor 21 and a ball screwportion 23 formed on an outer surface of the shaft 13 provided betweenthe steered wheels are screwed together such that when the rotor 21rotates, the ball screw portion 23 performs a linear motion. Further,the motor 19 is provided with a rotation position sensor 25 fordetecting a rotation position θ2 of the rotor 21. Moreover, as shown inFIG. 2, U, V and W phase windings 19U, 19V, 19W of the motor 19 arestar-connected, for example.

As shown in FIG. 1, a rack 24 is formed on one end portion side of theshaft 13 provided between the steered wheels, and a pinion 18 providedon a lower end of a steering shaft 16 meshes with the rack 24. Further,a steering wheel 17 is attached to an upper end of the steering shaft16, and a torque sensor 27 and a steering angle sensor 26 are attachedto an intermediate portion of the steering shaft 16. Furthermore, avehicle speed sensor 28 for detecting a vehicle speed V from a rotationspeed of the steered wheel 12 is provided in the vicinity of the steeredwheel 12.

A steering control apparatus 40 according to the invention is installedin the vehicle 10 to drive-control the motor 19. As shown in FIG. 2, thesteering control apparatus 40 includes a motor driving circuit 43 and asignal processing circuit 44, and when an ignition switch 94 is turnedon, the steering control apparatus 40 is connected conductively to abattery 92 and thereby activated. Further, the signal processing circuit44 includes a central processing unit (CPU) and a memory, not shown inthe drawing, and by executing a program stored in the memory, a driveswitching element group UH, UL, VH, . . . to be described below ison/off-controlled.

The motor driving circuit 43 is a three-phase bridge circuit in which U,V, W phase circuits 43U, 43V, 43W are provided between a positiveelectrode and a negative electrode shown here as GND of a boost circuit93 connected to the battery 92. A higher stage drive switching elementUH and a lower stage drive switching element UL are connected in seriesto the U phase circuit 43U, and the U phase winding 19U of the motor 19is connected to a power supply line 42U extending from a commonconnecting point of the two drive switching elements UH, UL via acurrent sensor 29 such as a Hall element. Similarly, a higher stagedrive switching element VH and a lower stage drive switching element VLare provided in the V phase circuit 43V, and the V phase winding 19V ofthe motor 19 is connected to a power supply line 42V extending from acommon connecting point of the two drive switching elements VH, VL viathe current sensor 29. A higher stage drive switching element WH and alower stage drive switching element WL are provided in the W phasecircuit 43W, and the W phase winding 19W of the motor 19 is connected toa power supply line 42W extending from a common connecting point of thetwo drive switching elements WH, WL via the current sensor 29. Further,the drive switching elements UH, UL, VH, . . . are constituted by Nchannel type MOSFETs, for example, and gates of the MOSFETs areconnected to the signal processing circuit 44.

An emergency switch series circuit 53 is provided at a midway point oneach of the V phase and W phase power supply lines 42V, 42W of the motordriving circuit 43. Each emergency switch series circuit 53 is formed byconnecting a pair of emergency switching elements 51, 52 in series. Inthis embodiment, N channel type MOSFETs are used as the emergencyswitching elements 51, 52. For example, sources of the two emergencyswitching elements 51, 52 are connected to each other such thatparasitic diodes 51D, 52D are oppositely oriented.

Hereinafter, a current flowing to the motor 19 from the motor drivingcircuit 43 will be referred to as a “positive current”, and a currentflowing to the motor driving circuit 43 from the motor 19 will bereferred to as a “negative current”. When the pair of emergencyswitching elements 51, 52 are to be differentiated, the emergencyswitching element having the parasitic diode 51D that is capable ofconducting the negative current will be referred to as a first emergencyswitching element 51 and the emergency switching element having theparasitic diode 52D that is capable of conducting the positive currentwill be referred to as a second emergency switching element 52. Further,the parasitic diodes 51D, 52D shift between a conductive state and anon-conductive state according to the orientation of the current, andtherefore function as switches. In consideration of this, a state inwhich the parasitic diodes 51D, 52D are conductive will be referred toas “on”, and a state in which the parasitic diodes are non-conductivewill be referred to as “off”.

The emergency switching elements 51, 52 receive a gate voltage from thesignal processing circuit 44 when the ignition switch 94 is turned on,and are thereby held in an on state. Hence, similarly to the powersupply line 42U not having the emergency switch series circuit 53, acurrent can be passed through the power supply lines 42V, 42W. Further,when the ignition switch 94 is turned off, the emergency switchingelements 51, 52 are turned off. When an abnormality occurs, on the otherhand, the gate voltage applied to the emergency switching elements 51,52 from the signal processing circuit 44 is stopped at a predeterminedtiming even though the ignition switch 94 is on, and as a result, theemergency switching elements 51, 52 are both turned off.

If the emergency switch series circuit 53 is provided with only oneemergency switching element, a current can flow in a single direction ofthe power supply line 42V, 42W via the parasitic diode even when theemergency switching element is off. In this embodiment, however, thepair of emergency switching elements 51, 52 is provided on therespective power lines 42V, 42W and the parasitic diodes 51D, 52Dthereof are oppositely oriented. Therefore, when both of the emergencyswitching elements 51, 52 are turned off, the power lines 42V, 42W entera completely non-conductive state. Accordingly, the two phase windings19V, 19W of the U, V and W phase windings 19U, 19V, 19W in the motor 19are blocked to become incapable of conduction, and as a result, themotor 19 and the motor driving circuit 43 are electrically disconnected.

After the emergency switching elements 51, 52 are turned on, the signalprocessing circuit 44 executes a steering control program (not shown)stored in the memory, not shown in the drawings, repeatedly to determinea motor current command value on the basis of respective detectionresults V, θ1, θ2, and Tf from the vehicle speed sensor 28, steeringangle sensor 26, rotation position sensor 25, and torque sensor 27. U,V, W phase currents Iu, Iv, Iw based on the motor current command valueare then determined on the basis of a detection value of the rotationposition sensor 25, whereupon the drive switching element group UH, UL,VH, . . . of the motor driving circuit 43 is on/off-controlled such thatthe U, V, W phase currents Iu, Iv, Iw are passed through the powersupply lines 42U, 42V, 42W of the motor driving circuit 43.

More specifically, the drive switching element group UH, UL, VH, . . .of the motor driving circuit 43 is set in two patterns, namely a patternin which any one of the higher stage drive switching elements UH, VH, WHis turned on and any two of the lower stage drive switching elements UL,VL, WL are turned on, and a pattern in which any two of the higher stagedrive switching elements UH, VH, WH are turned on and any one of thelower stage drive switching elements UL, VL, WL is turned on. As anexample, broken line arrows on the motor driving circuit 43 shown inFIG. 3A show the U, V, W phase currents Iu, Iv, Iw of the motor drivingcircuit 43 when only the U phase drive switching element UH is turned onon the higher stage and only the V phase and W phase drive switchingelements VL, WL are turned on on the lower stage, for example. In thiscase, the U phase current Iu flows from the higher stage of the U phasecircuit 43U into the U phase winding 19U of the motor 19 via the powersupply line 42U, whereupon the U phase current Iu is separated into theV and W phase windings 19V, 19W of the motor 19 to form the V and Wphase currents Iv, Iw. Note that the U, V, W phase currents Iu, Iv, Iwthat actually flow through the power supply lines 42U, 42V, 42W aredetected by the current sensor 29, whereupon the U, V, W phase currentsIu, Iv, Iw are feedback-controlled by the signal processing circuit 44on the basis of detected current values thereof.

When the drive switching element group UH, UL, VH, . . . is switchedbetween the and off states, all of the drive switching elements UH, UL,VH, . . . are temporarily turned off to ensure that the higher stagedrive switching element UH (VH, WH) and the lower stage drive switchingelement UL (VL, WL) are not turned on simultaneously, thereby causing ashort circuit in the phase circuit 43U (43V, 43W). At this time,electric power stored in the phase windings 19U, 19V, 19W, as aninductor, of the motor 19 is discharged such that a regenerative currentflows to the motor driving circuit 43 and the motor 19. Specifically,when all of the drive switching elements UH, UL, VH, . . . are turnedoff after causing the U, V, W phase currents Iu, Iv, Iw indicated by thebroken line arrows in FIG. 3A to flow, a regenerative current indicatedby broken line arrows in FIG. 3B flows. More specifically, theregenerative current flows from the lower stage of the U phase circuit43U to the power supply line 42U and the U phase winding 19U of themotor 19 through the parasitic diode of the U phase lower stage driveswitching element UL. The resulting U phase current Iu is then separatedinto the V and W phase windings 19V, 19W of the motor 19, after which itflows from the motor 19 through the power supply lines 42V, 42W and thenflows to the boost circuit 93 through the parasitic diode of the higherstage drive switching element VH of the power supply line 42V and theparasitic diode of the higher stage drive switching element WH of thepower supply line 42W.

The signal processing circuit 44 of this embodiment performs aninterruption to execute an emergency power supply stopping program PG1shown in FIG. 5 when an abnormality occurs in the vehicle speed sensor28 or the like, for example. The constitution of the emergency powersupply stopping program PG1 will be described below together with theinteractions and effects of this embodiment.

Next, the interactions and effects of this embodiment will be described.In the steering control apparatus 40 of this embodiment, during normaltravel of the vehicle 10, a three-phase alternating current constitutedby the U, V, W phase currents Iu, Iv, Iw generated by the motor drivingcircuit 43 is conducted to the motor 19 through the emergency switchingelements 51, 52, thereby electrifying the motor 19 such that the motor19 is driven to assist steering. Further, when an abnormality occurs inthe vehicle speed sensor 28 or the like, the steering control apparatus40 performs an interruption to execute the emergency power supplystopping program PG1 shown in FIG. 5. Upon execution of the emergencypower supply stopping program PG1, first, the drive switching elementgroup UH, UL, VH, . . . of the U, V, W phase circuits 43U, 43V, 43W inthe motor driving circuit 43 are all turned off in S10.

However, the aforementioned regenerative current or a current producedby power generation in the motor 19, for example, may continue to flowto the power supply lines 42U, 42V, 42W even after the motor drivingcircuit 43 has been stopped. If the emergency switching elements 51, 52are turned off at the same time as the power supply is stopped by themotor driving circuit 43, a spike voltage is generated, and as a result,the emergency switching elements 51, 52 may be damaged.

Hence, in the emergency power supply stopping program PG1, adetermination as to whether or not the power supply lines 42U, 42V, 42Ware conductive is made on the basis of the detection result of thecurrent sensor 29 after all of the drive switching elements UH, UL, VH,. . . have been turned off in S11. More specifically, for example, whenan effective value of the V phase current Iv is smaller than or equal toa predetermined reference value, it is determined that the power supplyline 42V is substantially non-conductive, and when the effective valueof the V phase current Iv is greater than the predetermined referencevalue, it is determined that the power supply line 42V is conductive.When the power supply lines 42U, 42V, 42W are non-conductivecorresponding to “NO” in S11, all of the emergency switching elements51, 52 provided on the power supply lines 42V, 42W are turned off inS12, whereupon the emergency power supply stopping program PG1 isterminated.

When the power supply lines 42U, 42V, 42W are conductive as “YES” inS11, on the other hand, a determination is made as to whether or not theV phase current Iv is substantially 0, or in other words no greater thana predetermined reference value in S13. When the V phase current Iv issubstantially 0 as “YES” in S13, the first and second emergencyswitching elements 51, 52 of the V phase power supply line 42V areturned off in S14. When the V phase current Iv is not 0 as “NO” in S13,on the other hand, a determination is made as to whether or not the Vphase current Iv is positive in S15. Note that a “positive” currentindicates a current that flows to the motor 19 from the motor drivingcircuit 43 and a “negative” current indicates a current that flows inthe opposite direction, as described above.

When the V phase current Iv is a negative current as “NO” in S15, thefirst emergency switching element 51 of the V phase power supply line42V is turned off in S17. When the V phase current Iv is a positivecurrent as “YES” in S15, the second emergency switching element 52 ofthe V phase power supply line 42V is turned off in S16. In other words,when the V phase current Iv flowing through the V phase power supplyline 42V is a positive current, only the MOSFET having the parasiticdiode 52D that is turned on by a positive current, from among theMOSFETs serving as the pair of emergency switching elements 51, 52provided on the V phase power supply line 42V, is turned off. Thus, asituation in which the current is blocked rapidly following turning offis prevented, and as a result, a spike voltage is not generated.

After one or both of the first and second emergency switching elements51, 52 of the V phase power supply line 42V have been turned off in thismanner, a determination is made as to whether or not the W phase currentIw is substantially “0” in S18. When the W phase current Iw issubstantially 0 as “YES” in S18, the first and second emergencyswitching elements 51, 52 of the W phase power supply line 42W areturned off in S19. When the W phase current Iw is not 0 as “NO” in S18,on the other hand, a determination is made as to whether or not the Wphase current Iw is positive in S20. When the W phase current Iw becomes“positive” as “YES” in S20, the second emergency switching element 52 ofthe W phase power supply line 42W is turned off in S21. In this casealso, a spike voltage is not generated, similarly to the case in whichthe second emergency switching element 52 of the V phase power supplyline 42V is turned off. When the W phase current Iw is “negative” as“NO” in S20, the first emergency switching element 51 of the W phasepower supply line 42W is turned off in S22.

After one or both of the first and second emergency switching elements51, 52 have been turned off in S19 or S21 or S22, a determination ismade as to whether or not all of the V phase first and second emergencyswitching elements and W phase first and second emergency switchingelements are off in S23. When any one of the emergency switchingelements is not off as “NO” in S23, the routine returns to step S13, andwhen all of the emergency switching elements are off as “YES” in S23,the emergency power supply stopping program PG1 is terminated.

When the emergency power supply stopping program PG1 shown in FIG. 5 isexecuted by the signal processing circuit 44 in this manner, the MOSFETsthrough which no current is flowing or the MOSFETs through which acurrent oriented to turn the parasitic diodes 51D, 52D on is flowing aresequentially turned off at the time of an abnormality such that all ofthe MOSFETs serving as the emergency switching elements 51, 52 areturned off. In other words, in this embodiment, the signal processingcircuit 44 during execution of the emergency power supply stoppingprogram PG1 corresponds to an “emergency turn off control unit”according to the invention.

When the emergency power supply stopping program PG1 is terminated, thepower supply lines 42V, 42W corresponding to two phases are disconnectedsuch that a conductive closed circuit partially including the phasewindings 19U, 19V, 19W of the motor 19 ceases to exist. Here, bymodifying the disposition of the emergency switch series circuit 53 suchthat the emergency switch series circuit 53 is provided at a midwaypoint on a trunk line 43Z connecting the motor driving circuit 43 to theboost circuit 93, as shown in FIG. 4A, for example, a current headingtoward the boost circuit 93 through the parasitic diodes of the driveswitching element group UH, UL, VH, . . . can be blocked by theemergency switch series circuit 53 at a midway point on the trunk line43Z when all of the drive switching elements UH, UL, VH, . . . of themotor driving circuit 43 are switched off, as shown by the broken linesin FIG. 4A, for example. However, when the drive switching element UHprovided on the higher stage of the U phase circuit 43U is damaged suchthat the drive switching element UH is permanently on, as shown in FIG.4B, for example, a conductive closed circuit partially including thephase windings 19U, 19V of the motor 19 is formed, and therefore thepower generation resistance of the motor 19 cannot be removed. Bydisposing the emergency switch series circuit 53 in the manner of thisembodiment, on the other hand, a conductive closed circuit partiallyincluding the phase windings 19U, 19V, 19W of the motor 19 is completelyeliminated when the emergency switching elements 51, 52 included in theemergency switch series circuit 53 are turned off, and therefore thepower generation resistance of the motor 19 is removed, enabling areduction in steering resistance at the time of an abnormality.

The interactions and effects of the embodiment described above can besummarized as follows. In the steering control apparatus 40 according tothis embodiment, the emergency switching elements 51, 52 are MOSFETs,and therefore a situation in which foreign matter infiltrates amechanical switch, causing the mechanical switch to malfunction, can beavoided. Hence, the motor driving circuit 43 and the motor 19 can bedisconnected reliably when an abnormality occurs. Further, the MOSFETsthrough which no current is flowing or the MOSFETs through which acurrent oriented to turn the parasitic diodes 51D, 52D on is flowing aresequentially turned off, the emergency turn off control unit turn offall of the MOSFETs when an abnormality occurs 51, 52, and therefore asituation in which a current is blocked rapidly can be avoided, therebypreventing generation of a spike voltage. As a result, expensive MOSFETscapable of withstanding a spike voltage need not be used as theemergency switching elements 51, 52, enabling a reduction in cost. Inother words, according to this embodiment, a steering control apparatus40 capable of disconnecting the motor driving circuit 43 and the motor19 reliably when an abnormality occurs can be provided at low cost.

A second embodiment of the invention shown in FIGS. 6 and 7 isstructured such that one emergency switching element 59 is connected inseries to each of the power supply lines 42U, 42V, 42W of the motordriving circuit 43. The emergency switching element 59 is an N channeltype MOSFET, and parasitic diodes 59D, 59D, 59D of the emergencyswitching elements 59 provided on the respective power supply lines 42U,42V, 42W are disposed in an identical orientation relative to the motor19. More specifically, in this embodiment, drains of the MOSFETs servingas the emergency switching elements 59 are connected respectively to thephase circuits 19U, 19V, 19W of the motor 19 such that only a currentflowing to the motor 19 can be passed through the parasitic diode 59D.

Further, as shown in FIG. 7, in an emergency power supply stoppingprogram PG2 according to this embodiment, when the power supply lines42U, 42V, 42W are not conductive as “NO” in S11, the emergency switchingelements 59 provided on all of the power supply lines 42U, 42V, 42W areall turned off in S28, whereupon the emergency power supply stoppingprogram PG2 is terminated. In a case where the power supply lines 42U,42V, 42W are conductive as “YES” in S11, the emergency switching element59 of the U phase power supply line 42U is turned off in S22 when the Uphase current Iu equals or exceeds “0” as “YES” in S21, the emergencyswitching element 59 of the V phase power supply line 42V is turned offin S24 when the V phase current Iv equals or exceeds “0” as “YES” inS23, and the emergency switching element 59 of the W phase power supplyline 42W is turned off in S26 when the W phase current Iw equals orexceeds “0” as “YES” in S25. All other constitutions are identical totheir counterparts in the first embodiment, and therefore identicalreference symbols to those of the first embodiment have been allocatedto duplicate parts and duplicate description thereof has been omitted.Similar interactions and effects to those of the first embodiment areexhibited by the constitution of this embodiment.

The invention is not limited to the above embodiments, and embodimentssuch as the following, for example, are included in the technical scopeof the invention. In addition to the following embodiments, theinvention may be implemented after being subjected to variousmodifications within a scope that does not depart from the spirit of theinvention.

(1) In the first embodiment, the invention was applied to a steeringcontrol apparatus for a so-called rack electric power steering apparatusin which the tubular motor 19 is connected to the inter-steered wheelshaft 13 by a ball screw mechanism, but the invention may be applied toa steering control apparatus for a pinion electric power steeringapparatus in which the motor is connected to the inter-steered wheelshaft by a rack and pinion mechanism or a steering control apparatus fora column electric power steering apparatus in which a motor is connectedto a midway point of the steering shaft by a gear.

(2) The emergency switching elements 51, 52, 59 of the first and secondembodiments were constituted by N channel type MOSFETs, but may beconstituted by P channel type MOSFETs.

(3) The current sensor 29 of the first embodiment was constituted by aHall element, but may be constituted by a shunt resistor. Further, thedisposal location of the current sensor is not limited to the powersupply lines 42U, 42V, 42W, and as long as the respective phase currentsIu, Iv, Iw can be detected, the current sensor may be disposed on acircuit provided between the drive switching element group UL, VL, WLand the GND electrode, for example.

(4) In the first embodiment, the determination as to whether or not therespective phase currents Iv, Iw are no smaller than or equal to thepredetermined reference value, that is, the respective phase currentsIv, Iw are substantially equal to “0”, was made on the basis of thedetection value of the current sensor 29, but a determination as towhether or not absolute values of the respective phase currents Iv, Iware smaller than or equal to the predetermined reference value, that is,absolute values of the respective phase currents Iv, Iw aresubstantially equal to “0”, may be made on the basis of a detectionvalue of the rotation position sensor 25 provided in the motor 19.Further, in the second embodiment, the determination as to whether ornot the respective phase currents Iu, Iv, Iw equal or exceed thepredetermined reference value was made on the basis of the detectionvalue of the current sensor 29, but a determination as to whether or notabsolute values of the respective phase currents Iu, Iv, Iw equal orexceed the predetermined reference value, that is, absolute values ofthe respective phase currents Iu, Iv, Iw are equal or exceed “0”, may bemade on the basis of a detection value of the rotation position sensor25.

What is claimed is:
 1. A steering control apparatus comprising: a directcurrent power source installed in a vehicle; a three-phase alternatingcurrent motor serving as a driving source of an electric power steeringapparatus; a motor driving circuit provided between the direct currentpower source and the three-phase alternating current motor so as togenerate a three-phase alternating current from an output of the directcurrent power source and conduct the generated three-phase alternatingcurrent to the three-phase alternating current motor, and the motordriving circuit includes a plurality of drive switching elements; anemergency switching element which is provided in at least two phases ofa three-phase power supply line connected to the three-phase alternatingcurrent motor within the motor driving circuit, and which is turned offwhen an abnormality occurs such that the motor driving circuit and thethree-phase alternating current motor are disconnected, wherein theemergency switching element is a MOSFET, MOSFETs are provided in pairsin each of the at least two phases of the three-phase power supply line,and parasitic diodes of the pairs of MOSFETs are disposed in oppositeorientations to each other; and an emergency turn off control unitprogrammed to first turn off all of the plurality of the drive switchingelements when an abnormality occurs and then sequentially outputcommands that sequentially turn off the MOSFETs through which a currentoriented to turn the parasitic diodes on is flowing when the abnormalityoccurs, until all of the MOSFETs are turned off.
 2. The steering controlapparatus according to claim 1, wherein the motor driving circuitincludes the plurality of drive switching elements connected between thedirect current power source and a ground electrode, and the three-phasepower supply line is connected to a connection point between a seriallyconnected pair of the drive switching elements.
 3. The steering controlapparatus according to claim 2, wherein a U phase power supply line isconnected to the connection point between a pair of U phase driveswitching elements, a V phase power supply line is connected to theconnection point between a pair of V phase drive switching elements, anda W phase power supply line is connected to the connection point betweena pair of W phase drive switching elements.
 4. The steering controlapparatus according to claim 1, further comprising a current sensorprovided in the motor driving circuit, wherein the emergency turn offcontrol unit determines whether or not the MOSFETs are able to be turnedoff on the basis of the three-phase alternating current detected by thecurrent sensor.
 5. The steering control apparatus according to claim 4,wherein the emergency turn off control unit determines that the MOSFETsare able to be turned off when the three-phase alternating currentdetected by the current sensor is smaller than or equal to apredetermined value.
 6. The steering control apparatus according toclaim 1, further comprising a rotation position sensor to detect arotation position of the three-phase alternating current motor, whereinthe emergency turn off control unit determines whether or not theMOSFETs are able to be turned off on the basis of a detected valuedetected by the rotation position sensor.
 7. The steering controlapparatus according to claim 1, wherein the emergency switching elementdoes not include a mechanical switch.
 8. A steering control apparatuscomprising: a direct current power source installed in a vehicle; athree-phase alternating current motor serving as a driving source of anelectric power steering apparatus; a motor driving circuit providedbetween the direct current power source and the three-phase alternatingcurrent motor so as to generate a three-phase alternating current froman output of the direct current power source and conduct the generatedthree-phase alternating current to the three-phase alternating currentmotor, and the motor driving circuit includes a plurality of driveswitching elements; an emergency switching element which is provided ona three-phase power supply line connected to the three-phase alternatingcurrent motor within the motor driving circuit, and which is turned offwhen an abnormality occurs such that the motor driving circuit and thethree-phase alternating current motor are disconnected, wherein theemergency switching element is a MOSFET, the MOSFET is provided singlyin all three phases of the three-phase power supply line, and parasiticdiodes of all of the MOSFETs are disposed in an identical orientationrelative to the three-phase alternating current motor; and an emergencyturn off control unit programmed to first turn off all of the pluralityof the drive switching elements when an abnormality occurs and thensequentially output commands that sequentially turn off the MOSFETsthrough which a current oriented to turn the parasitic diode on isflowing when the abnormality occurs, until all of the MOSFETs are turnedoff.
 9. The steering control apparatus according to claim 8, wherein themotor driving circuit includes the plurality of drive switching elementsconnected between the direct current power source and a groundelectrode, and the three-phase power supply line is connected to aconnection point between a serially connected pair of the driveswitching elements.
 10. The steering control apparatus according toclaim 9, wherein a U phase power supply line is connected to theconnection point between a pair of U phase drive switching elements, a Vphase power supply line is connected to the connection point between apair of V phase drive switching elements, and a W phase power supplyline is connected to the connection point between a pair of W phasedrive switching elements.
 11. The steering control apparatus accordingto claim 8, further comprising a current sensor provided in the motordriving circuit, wherein the emergency turn off control unit determineswhether or not the MOSFETs are able to be turned off on the basis of thethree-phase alternating current detected by the current sensor.
 12. Thesteering control apparatus according to claim 11, wherein the emergencyturn off control unit determines that the MOSFETs are able to be turnedoff when the three-phase alternating current detected by the currentsensor is larger than or equal to a predetermined value.
 13. Thesteering control apparatus according to claim 8, further comprising arotation position sensor to detect a rotation position of thethree-phase alternating current motor, wherein the emergency turn offcontrol unit determines whether or not the MOSFETs are able to be turnedoff on the basis of a detection value detected by the rotation positionsensor.
 14. A steering control apparatus including a direct currentpower source installed in a vehicle, a three-phase alternating currentmotor serving as a driving source of an electric power steeringapparatus, and a motor driving circuit provided between the directcurrent power source and the three-phase alternating current motor so asto generate a three-phase alternating current from an output of thedirect current source and conduct the generated three-phase alternatingcurrent to the three-phase alternating current motor, the motor drivingcircuit including a plurality of drive switching elements, comprising:an emergency switching element which is provided on a three-phase powersupply line connected to the three-phase alternating current motorwithin the motor driving circuit, and which is turned off when anabnormality occurs such that the motor driving circuit and thethree-phase alternating current motor are disconnected, wherein theemergency switching element is a MOSFET, the MOSFET is provided singlyin all three phases of the three-phase power supply line and parasiticdiodes of all of the MOSFETs are disposed in an identical orientationrelative to the three-phase alternating current motor; and an emergencyturn off control unit to first turn off all of the plurality of thedrive switching elements when an abnormality occurs and thensequentially turn off the MOSFETs through which no current is flowing orthe MOSFETs through which a current oriented to turn the parasitic diodeon is flowing, and the emergency turn off control unit turns off all ofthe MOSFETs when the abnormality occurs.